6: Shrink your travel footprint

If you drive or fly a lot travel could be the largest part of your footprint.

Travel footprints are often dominated by car use but just one long flight could cause air travel to be the largest part of your footprint.

This step will analyse what a travel footprint is, how much we travel, the carbon intensity of travel and the steps we can take to reduce our emissions from different modes of transport.

Travel footprints

A person’s travel footprint is made up by the emissions from each mode of transport they use. This is generally dominated by driving, bus, rail and air travel. In each case emissions result directly from fuel combustion as well as indirectly from fuel production and vehicle manufacturing.

In 2005 the average American travel footprint looked something like this:

Clearly this footprint is dominated by the vehicle use, which is mostly passenger cars and light trucks. In 2005 personal vehicle use accounted for 5.7 t CO2e of an average American’s footprint. Of this vehicle footprint around 73% was the result of direct fuel combustion, 18% arose from fuel production and about 9% came from vehicle construction.

The breakdown of emissions between different modes looks like this:

Almost 90% of total emissions arose from vehicles while 11% came from air travel. Bus travel made up around 1% while rail was only a third of a percent. The dominance of vehicle travel is a result of the large distances Americans drive each year and the relatively low fuel economy of the American car fleet, which averaged just over 20 MPG in 2005. High car usage is also reflected in the relatively limited use of public transport.

Although the average American travel footprint is large compared to most other countries, the predominance of vehicle use is common most nations. Although rail and bus travel make up larger shares in European and Asian countries, vehicle emissions remain the dominant source of travel emissions in almost all countries.

Using a method like the one described in our calculation page you can calculate your own travel footprint. This will show you how your own footprint is divided between vehicle, bus, rail and air travel emissions. In order to reduce this footprint you will either have to reduce the distance you travel or travel less carbon intensively. Although this step will focus mostly on the latter, we will begin with the former.

How much we travel

The simplest and cheapest way to reduce your travel footprint is to travel less. Of course this is often much easier said than done. As people get wealthier they tend to travel more, particularly for leisure. In most countries the average distance travelled has increased quite consistently over time.

To analyse distance travelled we use the idea of passenger miles or passenger kilometres. A passenger mile is literally each mile travelled by a passenger. For example a bus which drives 100 miles with 10 passengers results in 1000 passenger miles. Using emissions per passenger mile or kilometre allows comparison between different forms of transport. This helps account for the very different number of people travelling in cars, buses, trains and planes.

If we take the US as an example average per capita passenger miles have increased from around 7,000 in 1960 to more than 18,000 in 2005. Graphically it looks like this: Although increases in the oil price and recessions have reduced travel for short periods, because this data is in five-yearly intervals the overall upward trend is unrelenting. This trend may have peaked in recent years, but only future data will verify that. Personal vehicles are obviously the major form of transport used, while flying is the fastest growing type of travel.

On average American travel more than people from most other wealthy countries. This is due to America’s size, the way it has developed and its relatively low fuel prices. We can see this by comparing land based travel between different nationalities.

In 2005 Americans travelled over 25,000 passenger kilometres per capita (pkm/cap) in cars, buses and trains. This was considerably more than people of other wealthy countries. Canadians travelled around 16,000 pkm/cap, Australians 14,000, the French and British 13,000, Germans 12,000 and the Japanese around 10,000 passenger kilometres each. So while people definitely choose to travel more as they get wealthier, the extent of that travel does still vary from place to place.

Travel patterns also vary enormously from person to person. Your own footprint will be unique depending on where you work, go to school, shop, do personal business, socialise and take your holidays. Travel conducted while at work is generally omitted from an individual’s footprint, as it is counted as part of the footprint of a product or service.

To reduce distance travelled you need to make changes in your travel patterns. Reducing commuting is likely to both reduce your footprint and increase your quality of life, so if at all possible it is a great place to start. Unfortunately, much like trips to school, commuting miles can be very hard to influence. Shopping more locally, less frequently and online can help reduce shopping miles.

The travel you have most control over is often you leisure miles. Taking local holidays and less frequent long distance trips can reduce the distance you travel greatly. Any avoided flights in particular can have a large effect on your footprint, simply because flying is such an efficient way of racking up miles.

If you are focussed on reducing your carbon footprint you will no doubt find ways to limit the amount you travel. You will probably also find that travelling less isn’t always practical or desirable. Commuting to and from work is necessary, while travelling to see friends and going on holiday is something most of us enjoy. So while reducing the distance you travel is an effective way to shrink your footprint, it can be very hard work. Travel less carbon intensively on the other hand can be an attractive way to reduce your travel footprint.

The carbon intensity of travel

The emissions caused by travelling a kilometre (or a mile) can be hugely different depending on how you travel it.

Take the example of a person travelling 10,000 kilometres each year, similar to the average Japanese citizen. If a person drove that distance by themselves in a large car each year it could result in as much as 5 t CO2e. In a small fuel-efficient car it would be more like 2 t CO2e. Whereas on a scooter, in a car full of passengers or on a light rail system it could be less than 1 t CO2e.

To be more precise we can compare different modes of transport by calculating their carbon intensity in grams of carbon dioxide equivalents per passenger kilometre (g CO2e/pkm). These emissions occur as direct emissions from fuel use, indirect emissions from fuel production and from vehicle manufacturing. They are calculated based on the average loading (the average number of people travelling) which in the case of car travel is 1.6 people.

Our descriptions of petrol vehicles are based on fuel economy of 15 MPG (15.7 l/100 km) for a large car, 25 MPG (9.4 l/100 km) for a typical US passenger car, 35 MPG (6.7 l/100 km) for a small European car, 45 MPG (5.2 l/100 km) for a hybrid, 50 MPG (4.7 l/100 km) for a 250cc bike and 80 MPG (2.9 l/100 km) for a 50cc scooter.

Despite using descriptive names like ‘Large Car’ vehicles are better defined by their fuel economy, as this determines the vast majority of their emissions through fuel use. Actual fuel economy achieved is estimated, rather than official test cycle results which are generally not representative of real world driving. In addition to petrol vehicles we also include figures for air travel, rail, buses and electric cars.

Our example looks like this:

The most striking thing about this chart is the huge range in emissions between different forms of transport. This offers great potential to reduce your footprint by changing the type of transport you use, despite the fact that they are not all substitutes for one another.

From the most the carbon intensive Large Car example down to the Eurostar Rail there are different factors that influence the carbon intensity of travel. The major ones are occupancy, the energy efficiency of the vehicle, the route travelled and the carbon intensity of energy supply.

Despite their using relatively similar diesel vehicles, local buses, coaches and school buses have hugely different emissions. This is largely as a result of different occupancy levels as well as the type of route travelled . A heavy petrol car, with poor aerodynamics and an older engine can use three times as much fuel as a modern hybrid over the same distance. For electrified transport, like rail, the carbon intensity depends greatly on how that electricity has been produced.

Before getting into more specific detail about different types of travel it is worth understanding the breakdown of emissions. Looking at our travel modes again we can show the split between direct fuel emissions, indirect fuel emissions and vehicle manufacturing.

For each form of travel that uses an oil based fuel direct emissions from combustion are the most important. Indirect emissions from fuel production are around a fifth of direct ones, although this rises for energy intensive production like in the case of oil from tar sands. For systems using grid electricity the indirect emissions of electricity generation are most important factor, as can be seen from the low-carbon intensity Eurostar Rail, which is mostly powered by French nuclear energy.

Vehicle manufacturing emissions are very small for busy public transport systems, but make up a larger share of emissions in low-carbon vehicles. Because manufacturing emissions are based on the vehicle’s construction footprint divided by average lifetime mileage, both these factors can affect the estimate. Large Car manufacturing emissions are not always as big as expected because of high lifetime mileage, while Scooter manufacturing emissions may not be as small as one might think because of low lifetime mileage. The relatively large construction emissions of Electric Cars reflect a combination of lower estimated lifetime mileage, due to range restrictions, and carbon intensive battery manufacturing.

The obvious omission from the chart above is biofuels. These have been excluded because technologies are so numerous, and not universally available. Compared to petrol the emissions from biofuels vary enormously. Ethanol from sugarcane, recycled vegetable oils and second generations biofuels can all yield substantial emission reductions compared to petrol. In contrast things like corn ethanol and soy biodiesel often perform less well. Perhaps even more importantly production of certain biofuels can have adverse effects on food prices and contribute to deforestation.

Having taken a broad look at how carbon intensive different modes of travel are, we will move on to discussing each in turn.

Driving

Private vehicles are by far the largest source of travel emissions. If you own a car and drive it regularly then vehicle emissions are likely be the dominant share of your travel footprint. To reduce your driving footprint you either need to drive less, drive more efficiently, changing your vehicle or share more journeys.

Driving Less

As we have already discussed, reducing travel is the cheapest and most effective way to minimise your travel footprint, but it can be very difficult depending on your travel patterns.

Aside from reducing your total mileage, limiting the number of short trips you make is a good way to improve your footprint. Short trips involve a lot of stopping, starting and parking, which makes them an inefficient use of fuel. Walking or cycling for short journeys is a simple, and often quite pleasant, way to reduce emissions.

A more extreme way to ensure you drive less is to simply to get rid of your car. Growing access to car share schemes in big cities is making this idea more viable for many people. This idea is particularly appealing to city dwellers who like to drive occasionally but do their normal commuting on public transport, bike or foot.

Improving fuel economy

We can often increase the fuel economy we achieve in numerous ways without changing vehicles.

Cars use energy to accelerate, brake, power gadgets and overcome resistance from the air and road. Reducing how much energy is used doing these things will improve the fuel economy of your car. This can be done by avoiding rapid acceleration and braking, ensuring tyres are correctly inflated, limiting unnecessary weight in the car, removing roof boxes if not used regularly, and avoiding traffic when possible.

In terms of highway miles the most important factor is how fast you drive. Combustion engine cars tend to be most efficient from 40-60 mph ( 64-96 km/h) in their highest gear. Increasing this to 70 mph (113 km/h) can reduce fuel efficiency by as much as 30%. So driving highway miles at a moderate speed in the highest gear tends to be the most efficient way to drive.

Changing Vehicles

As we showed earlier changing the vehicle you use can have a dramatic effect on your carbon footprint. To illustrate these reductions we will compare the emissions of different vehicles assuming only the driver is on board. We will also add a small diesel car, and vary the source of the electric cars electricity in order to compare emissions based on different generation technology.

Our new example looks like this:

With only the driver to transport, both the motorbike and scooter are less carbon intensive per kilometre than all but the renewably powered electric car. For petrol cars carbon intensity falls as fuel economy rises, from 499 g CO2e/km for the 15 MPG car down to 190 g CO2e/km for the 45 MPG hybrid.

Once again it is useful to break these numbers down into direct emissions, indirect emissions and vehicle manufacturing.

From this breakdown we can see direct emissions still dominate vehicles using oil based fuels, but that manufacturing emissions become more and more important for low-carbon vehicles.

We can make these numbers more tangible by choosing an example. Let’s assume you drive 10,000 km a year by yourself. In the large car this will result in 5 t CO2e, in the medium car 3 t CO2e, in the hybrid or small diesel 2 t CO2e, in the scooter 1 t CO2e and with the solar powered electric car around 700 kg CO2e. The scope to reduce emissions by changing vehicle is clear.

If you are looking at reducing your travel footprint but still want the convenience of owning a vehicle, the main options are petrol, diesel, hybrid, electric or motorbike. Once again we will not discuss biofuels as their availability and carbon intensity vary greatly depending on location and technology used.

Petrol: Modern efficient petrol engines combined with lightweight materials and aerodynamic design can reduce vehicle emissions when compared to older vehicles. Despite this, petrol vehicles still tend to have higher carbon emissions than similar model diesels or petrol hybrids.

Diesel: Compared to a similar petrol car a diesel model will generally have carbon emissions around 15% lower. Diesels are hugely popular in Europe and used by many manufacturers to meet fleet emission targets forced on them by the European Union. Although lower in greenhouse gas emissions diesels generate more local air pollutants, like NOx emissions and particulates, which affect human health. In terms of climate change they also produce significant amounts of black carbon, which has a warming effect. These problems are reduced in new vehicles by filters which reduce particulate emissions by more than 80%.

Hybrid: Since the worldwide launch of the Toyota Prius in 2001 hybrid vehicles have become much more common, and are offered by numerous manufacturers. The hybrid vehicles we most commonly think of are parallel hybrids, which use petrol but also have an electric motor and battery. This motor both charges the battery and powers the car at low speeds. Petrol hybrids reduce emissions by around 20% compared to similar petrol cars, and perform much better in terms of local air pollutants than diesels. Diesel hybrids and plug-in hybrids are also gradually coming to market.

Electric: The carbon intensity of electric car travel depends greatly on the type of electricity used. Based on an electric car that uses 200 Wh/km, emissions vary from 252 g CO2e/km for coal generation to 166 g CO2e/km for natural gas to as little as 69 g CO2e/km for solar power. Vehicle manufacturing emissions make up 58 g CO2e/km of the total in each case. Even allowing for its greater manufacturing footprint, and lower lifetime miles, the solar powered electric car emits 60% less per kilometre than the diesel or hybrid example, making it by far the best option in terms of carbon emissions. The lack of tail pipe emissions also makes electric cars an attractive option for improving local air quality in cities. From a buyer’s perspective the main challenges are high purchasing cost, limited range and slow charging speeds.

Motorbikes: If you don’t mind being exposed to the weather motorcycles can be a very low-carbon form of travel. In our examples both the motorbike at 153 g CO2e/km and scooter at 101 g CO2e/km perform better than driving a hybrid or diesel on your one. This isn’t surprising considering the lighter weight and smaller engines of motorbikes, though they do often produce far more local air pollution. The low-cost of purchasing and running motorbikes makes them a very attractive option for many people. Electric motorbikes, scooters and bikes can provide even lower carbon travel, due to smaller manufacturing footprints and lower power needs.

Journey Sharing

Although it is quite obvious, the potential of sharing trips with friends and colleagues is very easy to overlook. Excluding the slight loss in efficiency due to extra weight, having a passenger in a car halves the carbon intensity of travel per passenger. If you had four people in a hybrid car this could result in emissions of less than 50 g CO2e/pkm, which is even lower than driving a solar powered electric by yourself, due to the sharing of total emissions among all four people.

Flying

The only real option for reducing emissions from flying is to fly less. While newer engines, lighter planes and certain types of biofuels do have the potential to improve the carbon intensity of flying, they are not things people have much control over.

The major problem with flying emissions is not the carbon intensity of flying but the great distances covered in short periods. In the example shown earlier a long distance economy flight emits 139 g CO2e/pkm, similar to a small car with average loading of 1.6 people. But a return economy flight from New York to London is roughly 7,000 miles (11,000 km), a distance comparable to an average driver’s yearly mileage. So the equivalent of a year’s worth of driving emissions can be created in just 15 hours of flying.

Any flight you avoid will significantly reduce your footprint, and if you fly often this will be the simplest way to make reductions. Between family, friends, and holidays this might not be easy, but it will be very effective. If you hope to reduce your personal footprint to a low target, like 4 t CO2e a year, there is very little room for flying at all, and certainly not for long distances. Given the lack of practical low-carbon substitutes for overseas flights this problem is not likely to go away any time soon.

Short flights (less than 3700 km) are often less carbon intensive than long flights (greater than 3700 km) because they generally have higher occupancy and lighter fuel loads. This doesn’t hold for very short flights (less than 1000 km) which are in fact more carbon intensive as they spend little time cruising, and are often not very direct. Business class flights for each distance are more than twice as carbon intensive as economy class due the extra space required.

Finally, it is important to note that in addition to its carbon footprint, flying has additional climate effects. Flights cause other positive and negative climate forcings, most notably through contrails, cirrus cloud formation and NOx emissions. Although these effects are much less well understood than those of carbon emissions, research indicates they result in a significant positive forcing (warming effect). This is often accounted for by using a multiplier. We do not use such a multiplier because doing so is inconsistent with the definition of a carbon footprint, and important short lived forcings are excluded from all other footprint calculations.

Public Transport

In most cases buses and trains are a less carbon intensive way to travel than driving, particularly if you often drive alone or in a large car. Just how carbon intensive they are depends on occupancy, fuel efficiency and carbon intensity of energy supply.

The example of US buses we showed earlier was 185 g CO2e/pkm for a local bus, 85 g CO2e/pkm for an intercity coach and 23 g CO2e/pkm for a school bus. Given that these are all diesel bus examples the main difference is occupancy, and to a lesser degree the nature of the route. These figures for diesel buses are roughly transferable to other countries. In places where occupancy is high, buses can generally deliver travel of around 100 g CO2e/pkm or better. Well occupied intercity coach travel is often as low as 50 g CO2e/pkm.

For transport that is electrified, whether it be trains, trams or buses, the carbon intensity is largely determined by the source of the electricity used. In our train examples we had 119 g CO2e/pkm for intercity rail in the US, 50 g CO2e/pkm for the New York Metro and 20 g CO2e/pkm for the Eurostar in France. Although occupancy and the efficiency of electric motors plays a part in these differences, the fuel mix of electricity is the main factor in this variation. In countries with low-carbon electricity like Brazil, France and Norway rail is a very low emission form of transport.

The impressive carbon intensity of public transport makes it a great option for reducing your footprint whenever practical.

Cycling and Walking

In comparing different modes of transport we have not yet considered cycling and walking as alternatives. This is mainly because the assumptions needed to estimate their carbon intensity are so problematic. As a general rule walking and cycling are very low-carbon ways to travel. They make especially good substitutes for short car trips, which are carbon intensive.

When we walk or cycle we use more energy than when resting, and this excess energy generally comes from our diet. By estimating the footprint of food used to supply this energy we can calculate a footprint for walking and cycling. For a person of average weight and average diet this footprint might be something like 40 g CO2e/pkm for walking or 20 g CO2e/pkm for cycling, including the manufacturing of the bike.

These estimates however have many problems. They require a number of assumptions about a person’s weight, fitness and diet that may not be valid. Even if these assumptions are reasonable at any given moment, in the long run a small amount of walking or cycling each day often reduces body weight, which in turn lowers baseline energy needs. Indeed in some cases constant moderate exercise could even lower your overall energy needs in the longer run.

If you did choose to calculate your cycling or walking emissions based on your diet, this should be done as part of your food footprint in order to avoid double counting. And if you are improving the carbon intensity of your diet in general, this would also serve to lower your walking and cycling footprints.

Summary

In this step we have looked at a typical travel footprint and the distances we travel. We have discussed ways in which we can travel less and compared the carbon intensity of different modes. We have focused on how to reduce your driving footprint, as well as looking at flying, public transport, cycling and walking.